1. Field of the Invention
The present invention relates to a high-strength steel plate used in constructing a storage container, more specifically to a steel plate excellent in low-temperature toughness of a heat affected zone (may be hereinafter referred to as “HAZ”) of a joint section when a multi-layer welded joint is formed.
2. Description of the Related Art
Because a thick steel plate used after being subjected to quenching and tempering (may be hereinafter referred to as “QT steel plate) has high strength and high toughness as well as excellent weldability, it has been conventionally used as a steel plate for a pressure vessel, mainly a tank. In recent years, such QT steel plate is widening its application area to a thick steel plate used for a storage container of a nuclear reactor and the like for example.
With respect to the thick steel plate applied to a storage container, there is a tendency that higher strength (585 MPa or above of the tensile strength, for example) is required in the light of the enlarged design of a welded structure in recent years. Further, because its construction is often planned in a cold area, the base metal and the welded joint section are required to have excellent low-temperature toughness.
For high strengthening of a thick steel plate applied to such application as described above, carbon of a specific amount or above (0.12 wt % or above, for example) is required. Also, when a welded joint is to be formed by welding thick steel plates, multi-layers are welded (multi-layer welded joint), however it is said that carbon content is preferable to be as less as possible in order to secure the toughness of such welded joint.
The structure of the multi-layer welded joint section shows a complicated aspect due to the construction of the joint. In other words, in the multi-layer welded joint section, it is known that tempered coarse grain HAZ (CG-HAZ) structure, tempered fine grain HAZ (FG-HAZ) structure, and two phase region heating HAZ (IR-CGHAZ) structure are present according to its position, and that the tempered CG-HAZ structure is the most embrittled section among these structures. That is, improvement of the toughness of the tempered CG-HAZ structure is considered to be the most effective means in order to make the low-temperature HAZ toughness of the welded joint excellent.
With respect to the cause of the tempered CG-HAZ structure becoming of low toughness (most embrittled section), presence of the grain boundary cementite in the tempered CG-HAZ structure can be cited. In other words, a crack is generated due to a fracture of the cementite present in the grain boundary of the structure, which means that the more the grain boundary cementite is, the more crack generation points are, and such fact is considered to be the cause of becoming of low toughness.
As a technology for obtaining a thick steel plate excellent in low-temperature toughness of the welded joint, a variety of proposals have been presented so far. As such technology, in Japanese Unexamined Patent Application Publication No. S63-103020 and Japanese Unexamined Patent Application Publication No. S63-169325 for example, a thick steel plate in which the toughness of the multi-layer welded joint is improved by controlling Nb content in the steel plate is proposed. The principle of the technology is that the island-shaped martensite (MA) in the tempered CG-HAZ is made easily decomposed in a succeeding weld pass.
However, even in such technology, the grain boundary cementite still remains even after MA has been decomposed, and from the viewpoint of improving low-temperature toughness by reducing the grain boundary cementite in the tempered CG-HAZ structure, the actual situation is that enough improving effect has not been exerted.